The CD5+ beta-lymphocyte subpopulation, which has been linked to several autoimmune diseases, has been reported to be increased in a subset of schizophrenic patients. To test the specificity of this finding, we measured the CD5+ beta-lymphocyte percentage of total lymphocytes in blood samples from 81 community mental health center patients and 10 controls. Results were analyzed according to diagnosis. The CD5+ beta-cell percentage in the substance abuser group was significantly elevated in comparison to all other groups. The percentage in the schizophrenic group was higher than the control group, but this increase was not statistically significant.